Sliding vane pumps are well known. Such pumps typically have a plurality of vanes slidably retained in radial slots of a rotor. The rotor has an axis about which it is rotated and which is eccentric to the axis of a cylinder in which the rotor is positioned. This creates a crescent shaped space between the rotor and the cylinder. When the rotor is rotated, the outer ends of the vanes follow the wall of the cylinder so that on one side of the rotor the pockets defined between the vanes increase in volume and on the other side of the rotor the pockets decrease in volume. An intake port of the pump is provided to the cylinder on the increasing side of the rotor, and an exhaust port is provided in the cylinder on the decreasing side. Thus, as the rotor is rotated, a gas or vapor is drawn into the intake and expelled through the exhaust.
In some applications of such a pump, for example as a vapor recovery pump for use in pumping liquid gasoline into the tank of an automobile at a gasoline station, the pump is required to operate pumping a vapor at sub-zero temperatures without seizing due to ice and frost accumulation inside the pump. To permit that, relatively large clearances are desirable. Such clearances are also desirable to reduce the failure rate due to inhaling debris.
However, a disadvantage of greater internal clearances is a reduction in the vacuum level capability of the pump. For example, in a gasoline vapor recovery system, it is known to sense the electric motor current which increases with increasing vacuum, and shut the system down when the current reaches a level that would indicate a blocked pipe. However, it is possible for the clearances inside the pump to be so great that such a high vacuum level cannot be reached, even if a pipe is blocked, so that the sensor does not perform its intended function.